Ongoing projects in our laboratory focus primarily on RNase A ribonucleases that promote host defense (the eosinophil ribonucleases, RNases 7 and 8, and leukocyte RNase A-2) with efforts directed toward understanding their biology, evolution and mechanism of antimicrobial action (Moser, Chan et al., in preparation). During this reporting period, we published the results of a study initiated by Dr. Wei Zhao during his sabbatical from Beijing Union College, Beijing, China, that was continued and completed as a result of the substantial efforts of a summer intern, Steven Siegel. In an effort to augment our current understanding of divergent mammalian RNase A ribonuclease sequences, we identified novel RNase 1 genes from four species of squirrel (order Rodentia, family Sciuridae). Squirrel RNase 1 genes encode typical RNase A ribonucleases, each with eight cysteines, a conserved CKXXNTF signature motif, and a canonical His(12)-Lys(41)-His(119) catalytic triad. Two alleles encode Callosciurus prevostii RNase 1, which include a Ser(18)<-->Pro, analogous to the sequence polymorphisms found among the RNase 1 duplications in the genome of Rattus exulans. Interestingly, although the squirrel RNase 1 genes are closely related to one another (77-95% amino acid sequence identity), the cluster as a whole is distinct and divergent from the clusters including RNase 1 genes from other rodent species. We examined the specific sites at which Sciuridae RNase 1s diverge from Muridae/Cricetidae RNase 1s and determined that the divergent sites are located on the external surface, with complete sparing of the catalytic crevice.